J. Phys. Chem. 1989, 93, 5265-5271
5265
Interactions of Ruthenium( II ) Photosensitizers with Surfactant Media‘ Seth W.
Scott L. Buell,2bJ. N. Demas,*vZband B. A. DeGraff*,2c
Department of Chemistry and Biophysics Program, University of Virginia, Charlottesuille, Virginia 22901, and Department of Chemistry, James Madison University, Harrisonburg. Virginia 22807 (Receiued: December 28, 1988; In Final Form: January 3, 1989)
Luminescent tris(a-diimine)ruthenium(II)photosensitizers exhibit complex emission intensity and lifetime curves when titrated with surfactants. The curve shapes depend on the charges and hydrophobicities of the complexes and surfactant. A simple model has been developed that accurately describes the interactions with anionic sodium dodecyl sulfate, nonionic Triton X- 100, and cationic cetyltrimethylammonium bromide. The model includes binding of the sensitizer to micelles and may also require formation of small premicellar aggregates. The strength of binding can be attributed to a combination of electrostatic attractions or repulsions and hydrophobic effects. Electrostatic attraction of oppositely charged species always yields tight binding. However, complexes with sufficiently hydrophobic ligands (Le., 4,7-diphenyl-l, 10-phenanthroline) can overcome electrostatic repulsions and bind to like-charged micelles. Binding of metal complexes to premicellar surfactant aggregates is normally stabilized by electrostatic interactions, and optimum aggregate size is close to the number required for charge neutralization. Results of this study can be used in the design of luminescent probes for specific microheterogeneous environments. Ruthenium(I1) complexes with cyano ligands are especially sensitive probes of local environment and show potential as luminescent probes for the rapid determination of critical micelle concentrations for anionic surfactants.
Introduction The interactions of tris(a-diimine)ruthenium(II) photosensitizers with micellar media have been an area of active r e ~ e a r c h . ~ The original goal of studying micelle-Ru(I1) interactions was to design photocatalytic systems, including solar energy con~ersion.~ Much effort was directed toward the control of excited-state quenching and subsequent thermal processes by the micelle environment.5 More recently, emphasis has shifted to utilizing Ru(I1) photosensitizers as probes of various heterogeneous and microheterogenous environments. Ionic Ru(I1)-a-diimine complexes can be designed that incorporate hydrophobic ligands and, thus, bind to crucial interface regions in microheterogeneous systems. If the polarity and hydrophobicity of a series of complexes are specifically tailored, partitioning of these probes between the bulk and microheterogeneous media can be investigated: thus making valuable local environmental probes for areas that cannot be monitored with bulk measurements. In our laboratory, we have undertaken a systematic characterization of transition-metal complex-micelle interaction^.^ W e have developed a D 2 0 method of measuring solvent accessibility, which is the degree of bulk solvent exposure of a bound probe.* We have studied the binding interactions with a variety of nonionic (1) Taken in part from: Snyder, S. W. M.S. Thesis, University of Virginia, 1985. Buell, S.L. Ph.D. Dissertation, University of Virginia, 1983. (2) (a) Biophysics Program. (b) Department of Chemistry, University of Virginia. (c) James Madison University. (3) (a) Kalyanasundaram, K. Photochemistry in Microheterogeneous and Heterogeneous Systems; Academic Press: New York, 1987; Coord. Chem. Reu. 1982, 46, 159. (b) Baxendale, J. H., Rogers, M. A. J. J . Phys. Chem. 1982, 86, 4906. (c) Foreman, T. K.; Sobel, W. M.; Whitten, D. G. J . A m . Chem. S o t . 1981, 103, 5333. (d) Schmehl, R. H.; Whitten, D. G. J . A m . Chem. S o t . 1980, 102, 1938. (e) Turro, N. J.; Yekta, A. J . A m . Chem. S o t . 1978, 100, 3931. (0 Meisel, D.; Matheson, M. S.; Rabani, J . J . A m . Chem. S o t . 1978, 100, 117. (4) Hautala, R. R.; King, R. B.; Kutal, C. Solar Energy: Conoersion Symposium; Humana: Clifton, NJ, 1979. ( 5 ) (a) Dressick, W. J.; Hauenstein, B. L., Jr.; Demas, J. N.; DeGraff, B. A. Inorg. Chem. 1984, 23, 1107. (b) Energy Resource through Photochemistry and Catalysis, Gratzel, M., Ed.; Academic Press: New York, 1983. (6) Mandal, K.; Hauenstein, B. L., Jr.; Demas, J. N.; DeGraff, B. A. J . Phys. Chem. 1983,87, 328. (7) (a) Snyder, S.W. M.S. Thesis, University of Virginia, 1985. (b) Buell, S. L. Ph.D. Thesis, University of Virginia, 1983. (8) (a) Hauenstein, B. L., Jr.; DreGick, W. J.; Buell, S. L.; Demas, J. N.; DeGraff, B. A. J . A m . Chem. Soc. 1983, 105, 4251. (b) Dressick, W. J.; Hauenstein, B. L., Jr.; Gilbert, T. B.; Demas, J. N.; DeGraff, B. A. J . Phys. Chem. 1984, 88, 3337. (c) Snyder, S. W.; Demas, J. N.; DeGraff, B. A. Chem. Phys. Lett. 1988, 145, 434.
0022-3654/89/2093-5265$01.50/0 , I
,
surfactants. We have employed sensitizer counterion effects9 and perturbations of quenching properties” to investigate sensitizermicelle binding. We correlate here binding studies in a variety of surfactant media. Our systems include cationic, neutral, zwitterionic, and anionic Ru(I1) complexes with anionic sodium dodecyl sulfate (SDS), nonionic Triton X-100 (TX-loo), and cationic cetyltrimethylammonium bromide (CTAB) micelles. We develop a binding model that accounts for behavior in both air- and N2saturated solutions. W e analyze the effect of ionic strength on the binding interaction. Where biphasic titrations are observed, we develop a model that accounts for premicellar interactions. Experimental Section The ligands and their abbreviations are as follows: 2,2’-bipyridine, bpy; l,lO-phenanthroline, phen; 5-methyl- 1,lOphenanthroline, Me-phen; 5-chloro- 1,10-phenanthroline, 5-CIphen; 5,6-dimethyI-l,lO-phenanthroline,5,6-Me2phen; 4,7-dimethyl- 1,lO-phenanthroline, 4,7-Me2phen; 3,4,7,8-tetramethylI , IO-phenanthroline, Me4phen; 5-phenyl- 1,10-phenanthroline, Ph-phen; 4,7-diphenyl- 1,lO-phenanthroline, Ph2phen; 4,7-diphenyl- 1,lO-phenanthrolinedisulfonicacid, ((Ph-S03)2phen)2-; and CN-. RuCl3 and all ligands were purchased from GFS Chemical Co. The syntheses of the complexes are described e l ~ e w h e r e . ~ ~ J ’ Mixed-ligand complexes of the form [RuL2L’I2+were synthesized in a two-step procedure via a RuL2C12intermediate.7a All cationic complexes were used as perchlorate or chloride salts, and all anionic complexes were used as potassium salts. Probe concentrations were kept in the range of 5-10 p M to minimize micelle multiple occupancy and self-quenching. SDS was purchased from Biorad and recrystallized from methanol. For low ionic strength measurements, an aggregation number of 62 was used.12 CTAB was purchased from Sigma and recrystallized from methanol. An aggregation number of 61 was used.12 TX-100 was purchased from Sigma and used as received. An aggregation number of 140 was used.13 Analytical (9) Hauenstein, B. L., Jr.; Dressick, W. J.; Gilbert, T. B.; Demas, J . N.; DeGraff, B. A. J . Phys. Chem. 1984, 88, 1902. (10) Snyder, S. W.; Raines, D. E.; Rieger, P. T.; Demas, J. N.; DeGraff, B. A. Langmuir 1985, I , 548. (11) Hauenstein, B. L., Jr.; Mandal, K.; Demas, J . N.; DeGraff, B. A. Inorg. Chem. 1984, 23, 1101. (12) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. ( 1 3) (a) Helenius, A,; Simons, K. Biochim. Biophys. Acta 1975, 415, 29. (b) Kushner, L. M.; Hubbard, W. D. J . Phys. Chem. 1954,58, 1163. (c) Law, K. Y. Photochem. Photobiol. 1981, 33, 799.
0 1989 American Chemical Society
5266
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989
1
Snyder et al.
T
" 1 :i!
v)
0 0.28
5 7 0 0
v
I;; 1
50 Q)
Y-
-1
400 0
30
20
10
50
40
SDS
0 16 0 00
60
mM
eters.
0
El .-0 1 1 0 E
v
.-J l.OO LC
1'
f
5 00
parameters,
'1
n
4 00
mM
Figure 4. Ru(phen)(CN),2- in CTAB, air saturated: see Table I for fit
25000
1.20
m
3.00
CTAB
Figure 1. Ru(bpy),2+ in SDS, N2 saturated: see Table I for fit param-
9)
2 00
1 00
I
m
6 20000
-Y
1 ' , TX-100 mM 5
0.90 0
10
CTAB
mM
Figure 2. Ru(phen)2((Ph-S0,)zphen) in CTAB, air saturated; see Table
5
0
20
15
15
10
20
25
Figure 5. Ru(phen)z(CN)2in TX-100, air saturated: intensity monitored: see Table I for fit parameters.
I for fit parameters, -0.30 0 9)
m
El
.-0 E
W
0.25
.--+ !i 9) .-1 *-
0.20 ~, 0
,
I
I
1
10
I
I
?
I
,
,
20
TX-100
,
I
,
I
30
40
I
mM
Figure 6. Ru(phen)(CN)t- in TX-100, air saturated; see Table I for fit
parameters.
2 7 0 ' 1 , 0
I
,
I
5
,
,
,
,
I
10
SDS
I
I
,
,
I
15
I
I
,
,
I
20
mM
Figure 3. Ru(phen),(CN), in SDS, (A) N2saturated and (B) air saturated: see Table I for fit parameters.
reagent-grade NaNO, and NaCl were used as received. Water was doubly distilled from alkaline permanganate. Lifetime measurements were carried out on a nitrogen laser system described e1~ewhere.l~Deoxygenation was accomplished
by bubbling with solvent-saturated N2,15and all measurements were made at 25 OC.I6 Steady-state emission measurements were carried out on an SLM 8000 spectrofluorimeter. Most luminescent decays were single exponential and were fit by a linear least-squares analysis of the semilogarithmic plot of intensity vs time.15 Double-exponential decays were fit by a Marquardt nonlinear least-squares method." (14) (a) Turley, T. J. M.S. Thesis, University of Virginia, 1980. (b) Turley, T. J.; Demas, J . N.;Demas, D. J . Anal. Chim. Acta 1987, 197, 121. (15) Buell, S.L.; Demas, J . N. Reo. Sci. Instrum. 1982, 53, 1298. (16) Buell, S. L.; Demas, J . N. Anal. Chem. 1982, 54, 1214. (17) (a) Marquardt, D. W. J . Soc. Ind. Appl. Math. 1963,11, 431. (b) Bevington, P. R. Data Reduction and Error Analysis for the Physical Sciences; McGraw-Hill: New York, 1969. (c) Demas, J. N. Excited State Lifetime Measurements; Academic Press: New York, 1983.
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989 5267
Interactions of Ru(I1) Photosensitizers with Surfactants The binding titrations were fit to the models by a simplex error minimization t e c h n i q ~ e . " ~ - Several '~ sets of initial guesses were tried in order to avoid trapping in local minima. At least five data points per floating parameter are required for reliable calculations. Fits were performed on an AT&T 6300 with an 8087 in Turbo Pascal.
DMS,. Thus, the binding constants for eq 4a and 4b should be similar. The free-donor binding constant, KDM, is given by
Results Luminescence Titrations. Upon addition of surfactant, the excited-state lifetime and luminescent intensity of many Ru(I1) complexes are altered. Figures 1-6 show the wide range of possible luminescent behavior observed. Titrations with a single curved region (monophasic) and two curved regions (biphasic) can occur depending upon the surfactant and probe; compare Figures 1 and 2. In addition, both increasing and decreasing trends in the luminescent intensity or lifetime can occur, even in the identical surfactant-probe system, by varying the oxygen concentration. As we will show, the shapes of these curves are controlled by a combination of electrostatic and hydrophobic interactions. Oppositely charged donor-micelle combinations show tight binding while like-charged donor-micelle combinations bind with greater difficulty. In general, increasing hydrophobicity of the ligands increases probe binding to all types of surfactant^.^ We have developed a binding model that accounts for all of these types of behavior. The model is general enough to be applied to cationic, anionic, and nonionic surfactants. We have used the model to successfully explain the titrations of numerous Ru( 11) probes under a variety of conditions.6
Since the donor is relatively large with respect to the micelle size, it can act as a nucleating site, and an induced cmc (icmc) must be emp10yed.l~
Model A complete binding model begins with interactions between the surfactant molecules (S) to form aggregates (S,) or micelles (M) of aggregation number N S,
+ S e S,+,
SN-I
**-*
+S
t)
M
D
+ S,
-
DS,
(3a)
DS,
(3b)
where DS, is an aggregate of n surfactant molecules and a donor. Potentially, a variety of paths can lead to aggregate formation, and these equations represent two extremes. For simplicity, we choose the limiting situation of eq 3a. However, both models give identical data fits; only the significance of the fitting equilibrium constants varies. Our model also includes donor-micelle association D
+M
Q
-
DM or D
DS,
+M
N
(6)
In the presence of premicellar aggregation, the binding constant KDs, is given by (7)
The fractions of free donor, fD, of donor-surfactant aggregate DS,, fDS,, and micelle-bound probe, fDM, are given by simple equilibrium expressions summarized in the supplementary material. The equilibrium concentration [SI is equal to the [SIformal below the icmc and is equal to the icmc when [SIformal is above it. These fractions can be related to our spectroscopic measurements. With our lifetime instrumentation, it is difficult to detect double-exponential behavior. The observed rate constant, kobs, for excited-state decay is given by a concentration-weighted average of the rate constants for the separate emitting components. Therefore, the observed lifetime, 'robs, is related to a weighted fraction of the bound and unbound rate constants
where the subscript on each T indicates the lifetime that would be measured for that species in the absence of exchange. Substituting in f s, we obtain
where [S]f,,l designates the formal surfactant concentration and cmc is the critical micelle concentration. Our model includes interactions between the photosensitizer (D) and either micelles or free surfactant molecules
+ nS
[SI - icmc [MI =
(1)
where the micelle concentration is given by
D
(5)
+ NS e DM
(4a)
DMS,
(4b)
where DM depicts a micelle-bound donor. The alternate form of eq 4a suggests the induced formation of micelles by the presence of the donor. We use only the first of these equations in our modeling and treat induced micelle formation by making the cmc an adjustable parameter, the icmc. We assume that the micelles interact similarly with free probes and small probe-surfactant aggregates. A micelle of aggregation number N approximates the average solution species, but the free surfactant species are in rapid equilibrium and enter and leave the micelle on a microsecond time scale.'* Since micelle size is distributed around N , there is little difference between DM and (18) Daniels, R. W. An Introduction to Numerical Methods and Optimization Techniques; North Holland: New York, 1978; Chapter 6.
Equation 9 is the general form of the binding equation in which interactions with surfactant aggregates of size n only have been included. The quality of titration data is inadequate to allow modeling to a distribution of different-sized surfactant aggregates. Equation 9 can be used to model a surfactant titration where robs is measured versus [SIformal.The unbound lifetime, i D , can be independently measured in the absence of surfactant. Simplex methods are used to fit the other six parameters: TDS., TDM,KDs,, KDM, n, and icmc. An important limiting case occurs where there is no premicellar aggregation. In that case, we only fit for TDM, KDM, and icmc. Equation 9 successfully fits all our monophasic and biphasic titrations. These titration analyses are not unique to lifetime changes. Luminescent intensity titrations, although generally slightly less reliable, can also be fit with eq 9.z0 However, intensity measurements can be useful when lifetime equipment is not available or when an experimental coincidence makes the lifetime changes small as compared to the intensity changes. Discussion We will show that all combinations of cationic, anionic, and neutral probes and surfactants are successfully fit by our model. Further, chemically reasonable parameters are obtained, and the results provide insight into the forces driving binding. In addition, we explore the impact of the microheterogeneous environment on (19) Mukerjee, P.; Mysels, K. J . A m . Chem. SOC.1955, 7 7 , 2937. (20) Dressick, W. J.; Demas, J. N.; DeGraff, B. A. J . Photochem. 1984, 24, 45.
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The Journal of Physical Chemistry, Vol. 93, No. 13, 1989
TABLE I: Binding of Ru(I1) Photosensitizers to Various Surfactantsa Ru(1l) complex purge gas surfactant T D , ns SDS 608 N2 SDS 890 NZ 429 SDS air air SDS 429 SDS air 199 air SDS 1 SDS 654 NZ air SDS 329 SDS 653 N2 SDS 3770 N2 SDS 560 N2 air CTAB 80 1 air CTAB 500 CTAB 1860 N2 CTAB 759 NZ air CTAB 902 CTAB 3403 N2 62 I CTAB N2 air 342 CTAB 1 air CTAB CTAB 548 N2 CTAB air 307 3769 CTAB N2 1100 TX- 100 N2 TX- 100 1 air 589 TX- 100 N2 309 TX- 100 air
TDS,
Snyder et al.
ns
270
328
0.944 424 251 427
-
TDM,
ns
821 I930 756 756 352 1.67 534 3 40 548 5270
nb
KDs,,, mM-" 0.029 -
6X -
0.0785 0.105 0.0283 0.359 -
K D ~mM-' ,
icmc, mM
265
5.40
1600 1600 24.1 44.7 3 20 289 538 31.1
0.59 0.59 7.44 7.52 7.71 7.60 2.62 I .54
-
I230 2350
458
4.00
252 0.73
0.57 0.83
-
153 44.1
0.255 0.823
-
168 19.7 -
-
-
1232 3867 2560 1080 6.10 452 250 8100 2010 4.43 288 186
-
7.30 10.9
-
-
3.66 11.8 295 345 126 17.0 18.5 12.5
0.0 0.48
0.75 0.1 1 0.46 0.008 1.31 0.56
"All measurements are by the lifetime method, unless otherwise specified. b T h e premicellar aggregate size S,, used in the fit of eq 9. 50 pM) compared to the virtually insoluble Ru(phen),(CN), or Ru(Ph,phen),(CN),. Its solvent accessibility of 0.478ashows that it is not as well shielded as the cationic probes. In Table I, we include entries for Ru(bpy),(CN), binding titrations monitored by lifetime and intensity methods. Little change was detected below the cmc. Both titrations yielded icmc of ~ 7 . mM, 5 which is near t h e literature values. This indicates little perturbation of micelle formation. In both titrations, similar K D M values were obtained. While there is a suggestion of a premicellar interaction, the changes are too small to allow accurate estimation of a Kw. This result suggests that without electrostatic attractions small polar probes will show only weak premicellar interactions and photophysical properties will be minimally perturbed. Ru@hen)2(CN),. Ru(phen),(CN), is a small n e u t r a l probe but is much less soluble in water than its bpy analogue ( T D M > T~~ in N2 and TDM > T~ > T~~ in air-saturated solutions. These (22) Lianos, P.; Zana, R. J . Phys. Chem. 1980, 84, 3339
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989 5269
Interactions of Ru( 11) Photosensitizers with Surfactants TABLE 11: Bindine of Ru(ohen),(CN), to SDS purge gas 'n T D , ns TDS, ns N, 1 654 324 2 654 424 N; 3 654 444 N2 4 654 452 N2 ~~
~
N2 air air air air air
5 1 2 3 4
5
654 329 329 329 329 329
455 0.026 25 1 269 275 277
TDM,
ns
535 534 535 531 53 1 340 340 340 340 340
KDS,, mM-"
KDM,mM-'
icmc, mM
0.114 0.105 0.0445 0.0178 0.00639 2.4 X IOd 0.0284 0.0121 0.00387 0.00 114
334 320 300 937 914 29 1 289 282 276 270
7.70 7.71 7.72 7.96 7.96 7.60 7.60 7.60 7.61 7.61
fDs,b at icmc 0.467 0.862 0.953 0.986 0.995 1.8 X 0.621 0.842 0.928 0.966
re1 error: %
1.59 1.29 1.38 1.78 2.20 0.938 0.995 1.06 1.21 1.36
'Premicellar aggregate size used in the fit to eq 10. bWeighted from calculated value with [SI = icmc and [MI = 0 (see supplementary material). CSum of the error residuals, calculated from re1 error = &,ints{(Yobs - Y,l,)Z/Yo,)(lOO%/points).
changes are a consequence of the differential O2 quenching of bound and unbound donors and demonstrate the usefulness of performing titrations under different atmospheres. Ru(phen),(CN), has large binding constants to both SDS micelles and premicellar aggregates under a variety of conditions. Titrations were performed in deoxygenated and air-saturated pure water and in deoxygenated 40 mM NaCl solutions. The KDM's of 320000 and 290000 M-' for N2- and air-saturated solutions, respectively, agree well. R ~ ( p h e n ) ~ ( C Nbinds ) , to SDS much tighter than R ~ ( b p y ) ~ ( c Nand ) ~ on the same order as R ~ ( b p y ) , ~ +This . larger binding cannot be explained by an electrostatic attraction. Rather, since R ~ ( p h e n ) ~ ( C N is )much ~ less water soluble than R ~ ( b p y ) , ~ + we, attribute the larger K D M to a hydrophobic interaction. Similarly, Ru(Ph2phen),(CN), is insoluble in water but very soluble in SDS and, therefore, has an effectively infinite SDS binding constant. Both titrations with R ~ ( p h e n ) ~ ( C N had ) ~icmc's near 7.7 mM, which is in good agreement with the literature value. The K D S , values of 0.1 1 and 0.028 mM-2 are substantial. The fits for the N2- and air-saturated data are presented in Figure 3. As with the R ~ ( p h e n ) , ~ + - s Dsystem, S changing ionic strength (40 mM NaCI) perturbs the R U ( ~ ~ ~ ~ ) ~ ( C N ) binding. , - S D S KDM is 538 000 M-I, but there is a very large uncertainty in this value because of the small post icmc T changes. The icmc of 3.2 mM agrees well with the literature value of 3.1 mM for a 30 mM NaCl solution.21 However, T~ and TDM are essentially unchanged in deoxygenated solutions with or without salt. Thus, ionic strength affects the driving force for binding but not the deactivation pathways. We turn to the question of the premicellar aggregation size (Le., n ) for the SDS interactions (Table 11). To do this, we varied n and examined the quality of the fits and the chemical plausibility of the calculated parameters. In both the N2- and air-saturated fits, the calculated micelle-bound lifetimes, TDM (531-535 ns for N 2 and 340 ns for air), are remarkably consistent. The titrations have plateaued, and this parameter is easy to determine. K D M is also very consistent for the different fits. Only in the N2saturated case with n = 4 does it deviate far from 300000 M-I. Additionally, the icmc remains in the expected 7.5-8.0 mM range. Except for the n = 1 case, T~~ is also consistent for the titrations (424-452 ns for N, and 251-275 ns for air). DS, is never a dominant species and, therefore, cannot be as well characterized. In the DSl case, our model explains the decreasing lifetimes below the icmc by assuming a small DS fraction with a very short lifetime. We cannot directly compare Kmn for different n's because the calculated binding constants are dimensionally different. For both titrations, the x: error remains relatively close for the fits over the range n = 1-3. The error starts to increase at n = 4 and becomes much larger at n > 4. This implies that the n = 1-3 fits are approximately equivalent in describing the titrations. This is not surprising as binding probably involves a range of aggregates from the monomer up to the micelle. However, the degradation of the fit for n > 3 indicates that a relatively narrow range of small aggregates dominates the mixture below the icmc. We stress that while the fitting procedure is tolerant of variations in n, some form of premicellar aggregation is essential to prevent catastrophic failure of the model.
R ~ ( ( P h - S 0 , ) 2 p h e n ) , ~R-~. ( ( P h - S O , ) ~ p h e n ) is , ~ the - largest and most anionic probe investigated. When bound to SDS, it has greater solvent exposure than the cationic and neutral complexes (F = 0.53).8a There was no evidence for premicellar aggregation. The K D M of 31 100 M-' is much weaker than either the cationic or neutral Ru(I1) complexes. The icmc of 1.5 mM again indicated probe-induced micelle formation. R~(phen)(CN),~-. R ~ ( p h e n ) ( C N ) , ~is- the only small anionic probe investigated. No binding to SDS was detected in deoxygenated or aerated solutions either by changes in T or by emission spectral shift on addition of S D S 7 * These results are expected on the basis of the electrostatic repulsion between the anionic probe and the micelle. While both are anionic, the charge is distributed very diffusely in Ru((Ph-SO,),phen),' in contrast to R ~ ( p h e n ) ( C N ) , ~ -The . cationic metal center is well separated from the anionic substituents, and electrostatic binding can occur by intercalation of the surfactant head groups between the sulfonated phenyl rings. With Ru(phen)(CN)2-, no large charge separation exists and interaction with the anionic surfactant is prohibited. Other Complexes. Measurements with SDS and the more hydrophobic cationic complexes such as Ru(5,6-Me2phen):+ and R ~ ( P h , p h e n ) , ~were + precluded. The interaction between the cationic probes and surfactant molecules in the premicellar region often irreversibly bleaches the solution. This detrimental phenomenon is probably due to microcrystallization of a Ru( 11)dodecyl sulfate ~ a l t . ~ ~ , ~ , ~ ~ Interactions with CTAB. CTAB has an aggregation number of 6 1 I 2 and a cmc of 0.98 mM.,' Previous luminescence studies in CTAB have focused on its interactions with nonpolar aromatic molecules,23rather than metal complexes.8c Several Ru(I1) probes exhibit binding with CTAB. Depending upon the complex, either monophasic (D and DM) or biphasic (D, DS,, and DM) behavior is observed. The fitting results are summarized in Table I. Unlike the anionic SDS, CTAB micelles have an electrostatic repulsion to cationic Ru(I1) probes. Therefore, interactions with these complexes must arise from the hydrophobic effects of the photosensitizer ligands in the alkyl core of the micelle. R ~ ( P h g h e n ) , ~The + . only cationic complex studied that exhibited a tight CTAB binding was R ~ ( P h ~ p h e n ) , ~the + , most hydrophobic cation probe employed. K D M for CTAB binding was at least a factor of 3 smaller than for binding to the neutral TX-100.6 R u ( M e ~ h e n ) , ~ 'The . hydrophobic R ~ ( M e , p h e n ) , ~ bound + weakly to CTAB. The K D M of 720 M-I was reduced by a factor of about 600 in comparison to TX-100 binding6 Interestingly, we observed that Ru(Me,phen),*+ formed premicellar aggregates, a phenomenon often observed in SDS interactions, but only otherwise observed in CTAB with anionic Ru(I1) complexes. This premicellar binding caused little change in lifetime ( T = ~ 500 ns vs T~~ = 458 ns). This interaction may be attributed to the balancing of two properties of R ~ ( M e , p h e n ) , ~ +It. is quite hy(23) (a) Atik, S. S : Singer, L.A. Chem. Phys. Lett. 1978, 59, 519. (b) Arora, J . P.S.; Singh, R. P.;Soam, D.; Singh, S. P.Bull. SOC.Chim. Fr. 1984, 1-19. (c) Wolff, T.; von Bunau, G . Ber. Bunsen-Ces. Phys. Chem. 1982,86, 225; [bid. 1984, 88, 1098.
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The Journal of Physical Chemistry, Vol. 93, No. 13, 1989
drophobic, and despite its hydrophobicity, it is a rather small complex. In contrast, the complexes with the very hydrophobic phenyl-substituted ligands show no biphasic behavior. We cannot preclude premicellar aggregation in these cases; if the lifetimes of D and DS, are similar, the titrations will show no evidence for the DS, species. Rubhen),( (Ph-SO,)zphen). As with the cationic complexes, neutral and zwitterionic Ru(I1) complexes tend to bind more weakly to CTAB than to either SDS or TX-100. R ~ ( p h e n ) ~ ((Ph-SO&phen) binds moderately strongly to CTAB micelles but shows no evidence for premicellar interactions. The K D M in the air-saturated solution is more accurate than that for the N2saturated solution because the lifetime changes are larger and the nitrogen-bubbled CTAB solutions are much harder to handle. A fit of the air-saturated titration is presented in Figure 2. Ru@hen),(CN),. The neutral complex, Ru(phen)z(CN)2,was also found to bind weakly to CTAB. Ru(phen)2(CN)zis very well shielded from solvent by the CTAB micelle.8c Although previous work has led us to assume that Ru(phen),(CN), must interact with some premicellar CTAB species,8cno lifetime or intensity changes were detected below the icmc. Since binding is not complete over the accessible CTAB concentration range, the titrations do not plateau and the KDM's cannot be measured very accurately. The variation of the intensity and lifetime K D M ' s is deemed to be within experimental error. CTAB binding causes the lifetime of R u ( ~ h e n ) ~ ( C N to )change ~ by a factor of 3 in aerated solutions. In N,-saturated solutions, the decays were complicated by non-single-exponential behavior over much of the useful CTAB concentration region. They were fit by the equation
where the M,'s and 7;s represent the weighting factors and lifetimes for the two decay components, respectively. Double-exponential analyses over a CTAB range of 3-6 mM gave an average short lifetime of 620 ns and a long lifetime of 2560 n ~ . ' The ~ short-lifetime component can be attributed to the unbound probe and the long lifetime to the bound p r ~ b e , ' ~ and , ' ~ thus, CTAB binding causes a factor of 4 change in lifetime. The larger change of a factor of 6 in I D M I I D in the intensity titration arose from a combination of two factors. First, increasing the lifetime increases the emission intensity. Second, the intensity titration was monitored near the maximum emission wavelength of the bound form, thus experimentally favoring I D M over I D . 7 a R ~ ( p h e n ) ( C N ) ~ ~The - . anionic R ~ ( p h e n ) ( C N ) , ~ bound strongly to the cationic CTAB in both the micellar and premicellar regions, although the solvent accessibility was large (0.6).8c The large K D M is consistent with a strong electrostatic attraction. As previously observed for cationic Ru( 11) complexes interacting with SDS, premicellar aggregates of the oppositely charged surfactant and probe essentially do not emit. Solutions frequently bleach irreversibly probably because of microcrystallization of probe-surfactant salts. The N2-saturated R ~ ( p h e n ) ( C N ) , ~ - CTAB solution lost all emission intensity in the premicellar region in titrations approached from either direction. In an air-saturated solution, we were able to carry out the titration through the premicellar region. Fits of the titrations usually gave very small values for T ~ and~ we, had to model with at least a DS, cationic species to obtain meaningful values for i D S . Results are presented in Figure 4. R ~ ( p h e n ) ( C N ) , ~did - not follow the normal lifetime trend for air-saturated solutions. In both air- and N,-saturated solutions, iD > TDM >> iDS. Ru(phen)(CN)z- was the only probe in which the lifetime decreased upon binding to CTAB (and also TX-100). Oxygen shielding is normally the largest factor associated with the lifetime increase upon micelle binding. Other factors, such as water accessibility and local solvent properties (Le., dielectric constant, viscosity, and rigidity), are also important. These properties must overwhelm the reduction in O2quenching upon binding. R ~ ( ( P h - S 0 , ) 2 p h e n ) , ~ -As . expected, the anionic Ru((PhSO,),~hen),~-interacted the strongest with CTAB. The lifetime
Snyder et al. plateaus to its completely bound value just above the icmc (lifetimes were unchanged above a formal surfactant concentration of about 1.5 mM). Although the fit for R~((Ph-SO,)~phen):was not as good as in some of the other systems, the data followed the common trend with iDM > iD> rDS. As with Ru(phen)(CN),-,, a cationic DS, species or larger was required to prevent iDS from becoming unreasonably small. Omitted Complexes. In general, the cationic complexes experience a strong electrostatic repulsion from the surface of CTAB micelles. Therefore, binding constants are much smaller than for SDS and are even smaller than for binding to the nonionic TX100.697aThis effect was so pronounced that a complex such as R ~ ( 5 , 6 - M e ~ p h e n ) , ~which + , bound tightly to TX-100 ( K D M = 57 000 M-1),6did not exhibit any interaction with CTAB. Similarly, Ru(phen),,+ and R~(S-Cl-phen),~+ show no evidence for binding. Interactions with TX-100.TX-100 is a polyether nonionic micelle-forming surfactant with N = 140 and cmc = 0.32 mM.', TX- 100 binding interactions with Ru(I1) probes have been covered p r e v i o ~ s l y . ~ In . ~ ~this ' ~ section, we investigate three additional complexes. R ~ ( p h e n ) ~ ( j - P h - p h e n ) For ~ ' . the mixed-ligand R ~ ( p h e n ) ~ (5-Ph-~hen)~',we found K D M = 126000 M-I. This K D M is intermediate in strength between that for the more hydrophobic R~(phen),(Ph~phen)~+ ( K D M = 333000 M-I) and less hydrophobic Ru(phen)32+( K D M = O).6 The lifetime increased upon binding, and the icmc was in the expected region. R ~ ( p h e n ) ~ ( C N ) ,Ru(phen),(CN), . binds weakly to TX-100. The intensity titration gave a large I D M I I D ratio of 4.4, but the lifetime titration exhibited double-exponential behavior and was not carried out in detail. The K D M of 17 000 M-I cannot arise from electrostatic attraction but must be due to a hydrophobic interaction with the sparingly water-soluble complex. The icmc was small, indicative of premicellar species or induced micelle formation. The best fit for the intensity titration is presented in Figure 5. R ~ ( p h e n ) ( C N ) , ~ -R, ~ ( p h e n ) ( C N ) , ~also binds weakly to TX-100. The K D M ' s in deoxygenated (18 500 M-*) and air-saturated (12 500 M-I) solutions indicate a similar strength binding as for the neutral Ru(phen),(CN)> As discussed with the CTAB interactions, T for Ru(phen)(CN),2- decreased upon binding to TX-100 in both the air and N 2 solutions. Also, as with Ru(phen),(CN),, the decays of Ru(phen)(CN),2- exhibit some double-exponential character. Best fit for the air-saturated titration is presented in Figure 6.
Conclusions While our equilibrium model utilizes a small number of species, it fits remarkably diverse systems, including all combinations of cationic, neutral, and anionic probes and surfactants. It provides insight into the solution composition above and below the cmc. This information would form the basis for interpretation of photochemical and thermal kinetics of micellar systems. Qualitatively, binding follows the expected trend based upon electrostatics: electrostatic attraction increases binding while repulsion decreases it. Compare Ru(bpy):+ and Ru(bpy),(CN)z in SDS where the electrostatic attraction increases the binding constant by at least 10-fold. Also, there is no detectible binding of the anionic R ~ ( p h e n ) ( C N ) , ~ to - SDS. Conversely, Ru(pher~)(CN),~binds tightly to CTAB while Ru(bpy),'+ shows no binding. Indeed, without electrostatic interactions, small polar cationic complexes (e.g., Ru(bpy),,+ or Ru(phen),2+) rarely interact with micelles. These complexes are normally the first investigated in surveys of luminescent probes, but we find them to be poor indicators of potential interactions between metal complexes and microheterogeneous environments. Binding shows a strong correlation with hydrophobicity in that complexes with very hydrophobic ligands bind more strongly. Compare R ~ ( p h e n ) ~ ( C N with ) , R ~ ( b p y ) , ( c N )in~ SDS where the more hydrophobic phen versus bpy ligand increases binding ~+ 10-fold. With CTAB, K D M follows the trend R ~ ( P h ~ p h e n ) ,>>
Interactions of Ru( 11) Photosensitizers with Surfactants Ru(Me4phen)2" >> Ru(phen)$+. These trends were also observed for binding to TX-100.6 Relatively modest structural changes can radically alter binding. The addition of one phenyl group to Ru(phen)3z+ converts a nonbinder on TX-100 to the strongly binding R ~ ( p h e n ) ~ ( 5 - P h - p h e n ) ~ + . For systems that can exhibit both hydrophobic and electrostatic interactions, there can be a balancing of the two effects. The reduction of binding due to electrostatic repulsions can be partially or largely offset by hydrophobic interactions. Consider Ru(phen)(CN)42-and Ru(Phzphen)32+in CTAB; the cationic and anionic complexes have virtually identical micelle binding constants. Thus, the hydrophobic interactions are powerful enough to overcome a strong electrostatic repulsion of the cationic complex and bind it as tightly as a strongly attracted, less hydrophobic, anionic species. These results show the relative importance of electrostatic and hydrophobic interactions and their effect on the strength of probe binding and provide semiquantitative data for the design of probes that will bind to different substrates or microenvironments. Our model and data clearly reveal the presence and importance of premicellar aggregates. Our luminescence data directly provide evidence for aggregates and their structure. In many cases, premicellar aggregates are essential for data fitting. Typically, aggregates are composed of a relatively small number of surfactant monomers associated with the probe. In particular, when the probe and monomer are of opposite charge, premicellar binding tends to be important and the aggregate sizes that give the best fits are close to those necessary for charge neutralization. Detectible premicellar aggregation is also common with the C N systems. The high environmental sensitivities of Ru(phen)2(CN), and Ru(bpy)2(CN)z,coupled with their small perturbation of the cmc for SDS, suggest that they may be useful as luminescent probes for rapid measurement of cmc's of anionic micelles. Our data for the other systems in which the icmc is much lower than expected were not taken in a manner that allows us to judge their suitability as cmc probes for these classes of micelles. In general, neutral complexes are more reliable for cmc determination. Oxygen quenching can be quite useful in separating T D and TDM (see Figure 3). Frequently, under one set of conditions the life-
The Journal of Physical Chemistry, Vol. 93, No. 13, 1989 5271 times are too close to allow parameter extraction, but by removing or introducing oxygen, the T'S can be separated and successful fits can be obtained. We stress that invariance of T before the cmc does not prove the absence of premicellar interactions. If association has little effect on the probe lifetime, our methodology fails to detect association. Indeed, using solvent exposure studies, we demonstrated the presence of premicellar aggregates in the CTAB-Ru(phen)z(CN)zsystem even though lifetime or intensity titrations fail to detect it.*' We point out that both the neutral and charged cyano complexes exhibit enormous environmental s e n s i t i ~ i t y . ~The ~ origins of this effect are unclear but probably involve specific interactions of the CN's with the local environment. This very high sensitivity is likely to provide cyano complexes with a unique role in the development of new probes of microstructure. Further work is in progress. Acknowledgment. We gratefully acknowledge the support of the National Science Foundation (Grants C H E 82-06279 and C H E 86-0012). All lifetime measurements were carried out on the University of Virginia laser facility purchased in part through NSF Grant C H E 77-09296. We thank K. Mandal for providing several titrations. Registry No. SDS, 151-21-3;TX-100, 9002-93-1; CTAB, 57-09-0; Ru(bpy)32+,15158-62-0; Ru(phen)32+,22873-66-1; R ~ ( b p y ) ~ ( c N ) ~ , 58356-63-1; R ~ ( p h e n ) ~ ( C N14783-57-4; )~, R~((Ph-SO~)~phen)~'-, 63244-81-5; Ru(phen)(CN)2-, 114737-30-3;R ~ ( P h ~ p h e n )63373~~+, 04-6; Ru(Me4phen)32+, 64894-64-0; R~(5,6-Me~phen)~~+, 14975-40-7; 63244-80-4; R~(S-Clphen),~+, 47860-47-9;R~(phen)~((Ph-SO,)~phen), R~(phen)~(5-Ph-phen)~+, 93503-35-6. Supplementary Material Available: Simple equilibrium expressions for the fractions of free donor, fD, DS,, fDsn, and micelle-bound probe,fDM (2 pages). Ordering information is given on any current masthead page. (24) (a) Kitamura, N.; Sata, M.; Kim,H.-B.; Obata, R.; Tazuke, S.Inorg. Chem. 1988,27,651. (b) Hinze, W.; Fendler. J. H.J . Chem. SOC.,Dolton Trans. 1915, 238.
